I have a psm bend in a channel shape. i then add a cut to one flange so its mitered. the results gives me a clean 45deg flanges edge. If i move the edge, then SE will recompute and add unwanted materail to my cut flange as shown in video.
I do not see any change when you drag the end of the flange. The cut area you highlight is the correct shape in my opinion. Remember that cuts which you apply in the bent 3D shape are calculated for the flat shape. That is the cuts are applied normal to the flat shape and not normal to the formed or bent shape. If they were calculated normal to the formed shape then you would have two edge profiles in the flat shape.
Flatten your part and you will see what I mean.
In addition you have control over how the cut is made. In the command bar you will see 3 options for the cut - "Thickness cut", "Mid-plane cut", and "Nearest-face cut".